"The bird flies."
Translation:पक्षी उड़ता है ।
है marks the sentence as being in the present tense. So, पक्षी उड़ता है is 'The bird flies' as opposed to 'पक्षी उड़ता था' which is in the past tense ('The bird used to fly').
Almost all present tense sentences in Hindi will have the है (or one of its other forms हैं, हूँ and हो). It can only be omitted in negative sentences. One other instance when present tense sentences don't have a है is when you have चाहिए.