What exactly does localisation mean?

Translation is about more than language alone.
Generally speaking, localisation means the fine-tuning of texts in accordance with local circumstances. For example, simply translating a contract is often insufficient, as the translation will also subsequently need to be checked to ensure that it satisfies local laws and procedures - work that usually requires a lawyer. Adjusting sizes and weights in a text also comes under the umbrella of localisation. When people refer to localisation in the translation sector, they are often referring to software localisation, ie. translating software and making it suitable for use in another country. This process requires more than purely translation work, such as adjusting the layout, help programs, codes, graphics, uniformity, etc.