I don’t think you can see it as an actual loan, because by supporting your project of choice getting a slot it helps encourage competition and the bestp ossibe allocation of fixed relay chain sources.
"These parachain slots are intentionally scarce to encourage competition and the best possible allocation of fixed Relay Chain resources. By limiting the number of available slots, aspiring parachains must outbid each other to earn a lease, both for the initial launch and again for renewal once the lease period expires. On Kusama, for example, these leases last a maximum of 48 weeks, while Polkadot has a maximum 96-week lease period.
This kind of competition and scarcity is an excellent way to help ensure that the most viable parachains occupy each slot: only successful parachains with traction, active usage, and a viable token economic plan can maintain a slot, while deserted or unsuccessful parachains cede their lease to more promising contenders."
In return for helping you get rewards from the project you’re supporting.
" 1. Community members temporarily lock their tokens (KSM for Kusama, DOT for Polkadot) in support of a particular project. These tokens are kept in a separate crowdloan module, which is on the Relay Chain and not controlled by the project. This helps prevent “rugpulls” by teams with malicious intentions.
2. Once the auction is open, the crowdloan module bids on behalf of the parachain project that it supports. Each parachain will be assigned with a campaign ID. Any DOT or KSM locked in this module will bid on behalf of the parachain. As long as an auction is occurring and the crowdloan is open, this bidding will continue to occur until a parachain slot is won.
3. Once a parachain slot is won, the parachain lease begins. If a slot is not won by the time the crowdloan ends, the tokens are unlocked and returned to their original owners. On Kusama, these leases began immediately after a slot was won. On Polkadot, these leases will begin in batches of five, with the first five parachains connecting to the network on December 17, 2021.
4. For parachains with successful crowdloan campaigns, the contributed tokens will remain locked on the Relay Chain until the lease ends. At that point, the tokens will be returned to the contributors. Most parachains so far have chosen to reward contributors with native project tokens, but those can be distributed throughout the lease period and are not tied to the parachain auction itself."