Taekwon-Do originated in Korea, and so the various moves and techniques we use have Korean names. Stands to reason, really. You will only be expected to know the Korean words that are applicable to your grade, i.e. those either listed on your grading sheet or applying to the techniques in your patterns or linework, however it is always a good idea to learn more than you need to know, just in case.
The dictionary tabs redirect you to the appropriate sections; these currently stand at over 560 words, so it might be an idea to use the search function on your browser unless you like scrolling through long lists; if this prospect is too appalling altogether, then just learn the ones on your grading sheets, and if there's anything else important it'll probably crop up in training at some point.
Finally, the other tabs in this section will show you how to count in Korean, and how to pronounce correctly the words you have learnt (with sound file links, to help ensure accuracy). Go to it!
An important note about pronunciation: All Korean terms as used in TKD are a 'best-fit' transliteration of the originals, which is to say that they are written as they should sound. Thus, the second syllable in 'Won-Hyo' should be pronounced with a leading 'H', and not as 'Yo'; and 'Yul-Gok' is pronounced with a final 'K' and not as 'Go'.
To add to the confusion (or fun, as we like to call it) a number of Korean sounds do not transfer exactly; thus some of the 'Si' words are pronounced as a very slight 'Sh' sound, such as in 'Sijak'; 'Sw' as in 'Swiyo' is pronounced more like 'Sh' with a very slight 'w' after the 'h'; the 'D' sound in e.g. 'Dan-Gun' is more of a cross between a 'D' and a 'T'; similarly the 'J' in 'Joon-Gun' is halfway between 'J' and 'Ch', and some 'G's (such as in 'Giokja') are a cross between 'G' and 'K'. Got it? Splendid!