Welcome to Gumnut!
On your turn you get the chance to play up to 4 of your cards but they have to match either the colour or the number of one of the four starting cards. You cannot mix numbers and colours. If you are trying to make a set of one colour then the numbers must all be different ( 1,2,3,4 ). If you complete a set of 4 then you take all four cards and keep them next to you.