An optimiser is a small device fitted to an individual panel that stops that panel from dragging down the rest of the string when it's shaded. On a normal string, all the panels are wired in series, so if one panel is in shade — from a chimney, a tree, or a nearby roof — its reduced output can throttle the entire string. Optimisers isolate each panel so a shaded one only affects itself, not its neighbours.
They're genuinely worth it where you have partial shading at some point in the day, or where you're mixing orientations on one string (for example five east-facing and five west-facing panels). The data shows that even minor shading on a single panel can have a surprisingly large impact on string output, so optimisers earn their keep in those situations. If your roof is completely clear with no shade and a single orientation, you may not need them at all.
A couple of practical tips: you can check whether optimisers are actually fitted by comparing the volts per panel against an un-optimised string — optimised panels typically read a few volts lower each. And if you're using Tigo-type optimisers, photograph and record each unit's serial number and location before install, in case you later add monitoring that needs to pair with them.