What are "prohibited / restricted areas" in the context of an Indian visa?

I doubt these are ones you might stumble across by accident. There are lots of details at immihelp, from which some extracted:

Andaman & Nicobar Islands
1). Municipal Area, Port Blair. 2). Havelock Island. 3). Long. 4). Neil Island. 5). Jolly Buoy. 6). South & North Cinque. 7).Red Skin. 8). Entire Island of center Andaman (excluding tribal reserve). 9). All Islands in Mahatma Gandhi Marine National except boat, Holoday, Twin Islands, Tarmugli Malay, and Pluto. 10). Entire Island of South Andaman (excluding tribal reserve). 11). Basatang Island. 12). North Passage Island. 13). Little Andaman Islands excluding tribal reserve. 14). Mayabunder. 15). Diglipur. 16). Rangat. 17). Mt. Harriet. 18). Madhuban. 19). Ross Island. 20).Narcondum Island. 21). Interview Island. 22). Brother Island. 23). Sister Island. 24). Barren Island restricted to visit on board vessels.

Arunachal Pradesh
Itanagar, Ziro, Along, Pasighat, Miao, Namdapha and Sujesa (Puki) Bhalukpong.

Himachal Pradesh
1. Poo-Khab-Sumdho-Dhankar-Tabo-Gompa-Kaza 2. Morang-Dabling

Kashmir
Khaltse Sub-Division: (Drokahpa Area)Khaltse-Dunkhar-Sroduchan Hanudo-Biana-Dha Nubra Sub Division: Leh-Khardung La-Khalsar-Tirit up to Panasik Leh-Khardung La-Khalsar up to Hunder Leh-Sabo-Digar La-Digar-Labab-Khungru Gampa-Tangar (Only for trekking conducted by approved tour operators and accompanied by State Police personnel) Nyona Sub Division: Leh-Upshi-Chusathang-Mahe-Puga-Tso-Moari Lake/Kozok Leh-Upshi-Debring-Puga-Tso-Moari Lake/Korzok. Leh-Karu-Chang La-Durbuk-Tangtse-Lukung-Spanksik. Pangong Lake up to Spanksik

Lakshadweep Islands
Bangaram, Suheli Islands.

Manipur
Lohtak Lake, Imphal, Moirang INA Memorial, Keibul Deer Sanctuary and Waithe Lake Kongjam War Memorial.

Additional areas opened up:

Imphal-Bishnupur-Phubala, Moirang-Sendra-Keibul-Lamjao-Imphal.
Imphal-Moirang-Lake Keibul-Lamjao-Chur-Chanderpur-Khuga-Imphal.
Imphal-Litan-Ukhral-Shiroy-Sangsak-Imphal.
Imphal-Thoubal-Waithu-Lake, Khonjam-Kakching-Thingjao-Waikhong-Sugnu-Imphal.
Imphal-Khonghampat-Kanglatombi-Kangpokpi-Senapati-Karong-Mao Gate-Imphal.
Imphal-Nonoh-Khongsang-Nungba-Jiribam-Imphal.

Mizoram
Vairangte, Thingdawl and Aizawl.

Nagaland
1. Dimapur District : Dimapur Town, Chumudima Sethikima and all places on the National Highway No. 39 en route to Kohima Town. 2. Kohima District : Kohima Town, Khonoma Dzulakie Kigwema , Jakhama Viswema, Khuzama, Japhfu, Dzuku Valley. 3. Mokochong District : Mokochong Town, Lungkhum, Ungma, Impur Mopungohukit, Chuchlyimlang, Tuli, Chani. 4. Wokha District: Wokha Town , Doyang , Vankhosand, Tsunki, Governor's Camp and Mount Tlyi.

Rajasthan
Western Side (up to Indo-Pak border) of NH No.15 running from Sriganganagar to Sanchore via Sriganganagar, Suratgarh, Lunkaransar, Bikaner, Gajner, Kolayat, Phalodi, Khera, RamDevra, Pokaran, Lathi, Jaisalmer, Sangad, Fatehgarh, Sheo, Bhadewa, Kapoordi, Barmer, Hathitala, Sanwara, Lookho, Gandhwa, Dhamuna subject to following exceptions: (1)The peripheral areas of cities, towns and villages on the NH N0. 15 through which it passes; (2) The city limits of Suratgarh, Bikaner, Phalodi, Pokaran, Jaisalmer, Barmer and Sanchore; (3) The following villages and areas of tourist interest in the district of Jaisalmer, Amarsagar, Ludrawa, Kuldara, Bada Bagh (black bar), Akal, Sam & Unda.

Sikkim
1. Gangtok, Rumtek, Phodang, Pemayangtse Khecheperi & Tashiging. 2. Zongri (West Sikkim) 3. Tsangu (Chhangu Lake in East Sikkim) 4. Mangan, Tong, Singhik, Chungthang, Lachung and Yumthang

Uttaranchal
1. Nanda Devi Sanctuary, Niti Ghati and Kalindi Khal in Chamoli, Uttar Kashi Districts. 2. Adjoining areas of Milam Glacier.

@jpatokal has recommended Wikipedia as easier to read.