What is IPA
The International Phonetic Alphabet (IPA) is a set of symbols where each one stands for a single sound. Dictionaries use it because spelling doesn’t always tell you how a word sounds: the letters “ough” are read differently in though, through and tough. In IPA each of them gets its own transcription — /ðəʊ/, /θruː/, /tʌf/ — so you can see the difference before you hear it.
How to read a transcription
Read the symbols left to right, one sound at a time. The mark ˈ comes right before the stressed syllable: in /ˈbɔɹdɪŋ/ the stress falls on “board”. The mark ː makes the vowel before it long, as in /pliːz/. For Russian and Ukrainian the tool also puts a stress mark over the vowel in the word itself. That matters: in Ukrainian за́мок is a castle and замо́к is a lock.
US and UK pronunciation: what changes
The most noticeable difference is the r. American English pronounces it after vowels, British English mostly drops it: water is /ˈwɔːtɚ/ in the US and /ˈwɔːtə/ in the UK. Some vowels change too. Pass is /pæs/ in American English and /pɑːs/ in British, sorry is /ˈsɑɹi/ and /ˈsɒri/. Choose the accent you want to sound like, and use Switch to compare the two on the same sentence.