"A kitten is very small."
Translation:Tha piseag glè bheag.
That is a huge question and it sounds as if you really need to read the Tips and Notes to see how huge.
A simplistic answer would be that the word glè causes words that follow it which begin with certain letters (including b) to acquire an h after that letter. This is called lenition. But there are many reasons in Gaelic why words may lenite and it's not straightforward, and a number of these are explained in the Tips and Notes.
There can be accessed from the access pane to each skill, but the entire set is available here on a single page for ease of reference. https://duome.eu/tips/en/gd
It would be helpful if Duolingo had Gaelic built out a little more. The tips are only accessible in a web based format instead of in each section like they are on other languages like Spanish and French, which is more convenient for reading them before starting a new topic. Thank you for linking to it here.