In Japanese, のは and のが behave in a very similar way to the words ‘that’ and ‘which’ in English. This means that they perform a task called nominalization. Nominalization is when a phrase is treated in the same way as a single noun.

• 沢山（たくさん）食（た）べるのは彼（かれ）です。
  He is the one that eats a lot. (Literally ‘the one that eats a lot, he is’)
• 私（わたし）はバスに乗（の）るのが嫌（きら）いです。
  I don't like riding the bus. (Literally ‘that which is riding a bus, I hate’)

Rather than a single noun, we can see from these examples that the phrases 沢山（たくさん）食（た）べるのは and バスに乗（の）るのが are behaving in the same way that a single noun would. In English, this can either mean ‘that which’, or ‘the one who/that’.
This construction can be used after verbs in any tense, except for the polite ます or ません, which are only used at the end of a sentence, or certain clauses.

• 彼（かれ）は走（はし）るのが好（す）き。
  He likes running.
• お菓子（かし）を全部（ぜんぶ）食（た）べたのは、娘（むすめ）だ。
  My daughter is the one that ate all of the candy.
• 山（やま）を登（のぼ）っているのは、今田（いまだ）さんだ。
  Imada-san is that one who is climbing the mountain.