Coming to base is getting your hips under you from the belly down position. Being in the belly position is a bad news. It allows your opponent to many opportunities to submit or strike you. You have to learn how to get out of it by regaining your base instead of pushing yourself up to your hands and knees – that would expose too many holes for your opponent to sink his hook and hands into to turn you over and submit you.
To come to base, first keep elbows in tight to your body. Then slide one of your knees towards the near-side elbow being careful to tuck the elbow tight inside the knee. Tucking makes it more difficult for your opponent to find holes. Once you have one knee under you, keeping your elbows tucked under your throughout, push your hips back over the other knee. By keeping the elbows tight and pushing your back over the other knee you have reclaimed your base