The hose from your portable AC connects to the slider to send the hot air emitted from the unit outside. You'll want to measure your window before purchasing your AC to ensure the slider will fit.

PK Accommodation Ltd is one of the UK’s largest providers of cheap refurbished portable accommodation ... are powered from an integral consumer unit, plumbing (where applicable) is in use ...

Using a portable AC unit is not only cheaper, it's also less of a hassle.