In a nutshell, gross bookings is the total Uber or Lyft charge from riders, before paying drivers, which is easily one of the biggest expenses for both companies. They both calculate the exact bookings line slightly differently (Uber includes Uber Eats, for example), but for showing directionality, it's a useful metric to investors.
"Our long-term valuation framework for Lyft assumes that the company achieves a 31.4% bookings share by 2029 (vs. an estimated 14.9% share today)," D. A. Davidson's White said in his initiation report.