The company registers a dollar amount of securities once and then takes them off the shelf as needed. Nothing is issued when the shelf is filed, which is why a shelf filing is not itself an offering, although the market often sells it as though it were.
The useful reading is capacity and intent. A large shelf filed by a company with two quarters of cash left is a plan. A routine renewal by a profitable issuer is housekeeping. Smaller companies face a baby shelf limit that caps sales at a fraction of their public float over any twelve months.
Example: a $750M shelf is filed. The company later draws $200M through an at-the-market-offering and $300M in a follow-on-offering, leaving $250M of capacity available without a new filing.
Related: at-the-market-offering, follow-on-offering, dilution, authorized-shares, use-of-proceeds