is the hard fork happens in a predefined intervals?

No it doesn’t. Monero hardforks are more commonly referred as “protocol upgrades,” and they are mainly aimed at improving Monero’s privacy tech.

In the past, some protocol upgrades were “predefined” in the sense that it would take place every 6 months. This was solely done in order to discourage the development of ASIC mining machines which grant disproportional advantage to the big players against the smaller mining enterprises in Monero’s mining network. However, this “predefined protocol changes to the mining algo” became moot after the introduction of RandomX mining algo, which is a CPU-supremacist algorithm for Monero production.

Could you please explain in easy terms what’s Seraphis and what changes it will bring?

Seraphis aims to make Monero’s codebase more modular. That way, future improvements to its privacy protocol is hoped to be introduced in a less-disruptive-way to the remainder of the codebase.

One additional protocol improvement that’s in the works is “Jamtis,” which changes the addressing scheme of Monero, in order to make it more flexible and fix some of its potential bugs. For more information see this presentation by Justin Berman: https://piped.video/watch?v=xGEBRQU1lzw

Lastly, very recently KayabaNerve announced that he had been working on Full Membership Proofs (FMP) for Monero, which finally has the potential to fix the weakest link in Monero’s privacy protocol: ring signatures. FMP, does away with the ringsigs and the decoy selection troubles that Monero has had up to today. With FMP, your transactions will reference the entire set of enote that Monero blockchain has, instead of a 16-, or 128-size other decoys.

All in all, quite good improvements are in the works for monero. Among them, I consider the Jamtis the most “disruptive” to the current users, as it will cause existing addresses unable to be used after the protocol upgrade. But fear not: if all goes well, you will be able to generate new Jamtis addresses using your existing wallets, and keep receiving 'neros to your wallet. And, of course, your existing funds will still belong to you and you only, nobody is going to lose any funds.