There is a great explanation of the definition of ostinato at Wikipedia.

In short:-

In music, an ostinato (derived from Italian: “stubborn”, compare English: obstinate) is a motif or phrase which is persistently repeated at the same pitch. The repeating idea may be a rhythmic pattern, part of a tune, or a complete melody.^[1] Both “ostinatos” and “ostinati” are accepted English plural forms.

Here are some simple steps to get the improvisation started at your piano:

1. Create an ostinato or riff on the left hand

Once the riff is created, the basic element of a song is alredy done. We have the tempo, the rhythm, the catchy motif, the key etc .. you name it. It is easy to start with C major, play chord C for 4 bars and then Chord F for 4 bars repetitively. Refer the video.

2. Improvise melody on the right hand

Just simply play what we hear in our mind. Hitting any of the white key should be fine.

Reference :

Left-Hand Ostinato Patterns and Why They’re Perfect for Piano Improvisation!

Ostinato from Wikipedia

Classical ostinato