Hunting NPC pirates (“rats”) is PvE content, searching through space for hostiles and destroying them for bounties and loot.
The primary places rats can be found are in asteroid belts and in Combat Anomalies. Combat Signatures can contain difficult sites full of tough rats as well, but those need a probe scanner to find, and thus require cooperation with an Explorer (or a scanning ship of your own).
Rats get progressively tougher - and with progressively better bounties - as you further down the scale of system security. In most highsec systems, ratting isn’t particularly worth bothering with, while in deep nullsec it can provide tens of millions of ISK per hour.
Asteroid belts are usually less lucrative, but (especially in nullsec) have the chance to spawn special “faction” or even “officer” rats, which drop extremely rare and valuable modules. The chance is low, but winning the asteroid-belt lottery can give a nice infusion of cash.
As the dangers from hostile players in nullsec are extreme, taking full advantage of nullsec ratting will usually require joining a nullsec corporation, so that you can live in their space unmolested.
Ratting can be a reliable moderate income stream, with occasional jackpots of faction modules that can sell for large sums of ISK.