Translation:Spring is March, April and May.
This is just a way to express «X is Y» in Ukrainian.
Ukrainian doesn’t use the verb 'to be, is' as often as English does, so it needs other means of marking «X is Y» sentences. So, «це» is introduced in place of «is». (From the grammar point of view, this sentence can be analyzed as 'Spring, this [is] March, April and May.')
It’s actually pretty common to put 'this' in «X is Y» sentences in different languages. For example, Mandarin Chinese did the same thing with 是 shì.