It tracks a certain calculation of inflation. There are youtubers who can calculate it out very precisely. But in general terms it tracks inflation, and resets every 6 months.

Every six months the rate changes. The rate of an I Bond is called the “combined rate”, as it’s based on a formula using an annual fixed rate and a semi-annual inflation rate. The formula is [fixed rate + (2 x semiannual inflation rate) + (fixed rate x semiannual inflation rate)].

The fixed rate has been and probably will remain 0%. The inflation rate is what has been fluctuating, and gets set every 6 months.