は (wa)

は is the topic particle. It tells the listener what the rest of the sentence is commenting on — “As for X, …”

私は学生です。
Watashi wa gakusei desu. — “I am a student.”

今日は天気がいいです。
Kyō wa tenki ga ii desu. — “The weather is nice today.”

Remember: the kana is は, but as a particle it is always pronounced wa.

For a full comparison with が, see は vs が.