what is tummy time ?

♡ 𝓚𝓪𝓽𝓻𝓲𝓷𝓪 𝓵𝓸𝓾𝓲𝓼𝓮 ♡
Things are different in the UK to America but on this app people recommend I do tummy time with my baby boy Jacob who is 3 weeks old, but what is it ?