A single kanji might have multiple on'yomi pronunciations, reflecting the Chinese pronunciations from different time periods or regions. On'yomi pronunciations are generally classified into go-on, kan-on, tō-on and kan'yō-on, roughly based on when they were borrowed from Chinese during the peaks of the Sinosphere.
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On reading in the context of Shinjitai
Shinjitai (Japanese: 新字体; 'new character form') are the simplified forms of kanji used in Japan since the promulgation of the tōyō kanji list in 1946. Some of the new forms found in shinjitai are also found in simplified Chinese characters, but shinjitai is generally not as extensive in the scope of its modification.
Shinjitai were created by reducing the number of strokes in kyūjitai ("old character form") or seiji (正字; "proper/correct characters"), which is unsimplified kanji (usually similar to traditional Chinese characters). This simplification was achieved through a process (similar to that of simplified Chinese) of either replacing the onpu (音符, "sound mark") indicating the on reading with another onpu of the same on reading with fewer strokes, or replacing a complex component of a character with a simpler one.
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