The root of a word is the part that contains the lexical (as against grammatical) meaning. For example, the root of the word вхћдят is ХОД 'go, come, walk', while в- is a prefix and -ят is a grammatical ending.
A root usually consists of two consonants with a vowel in between. The consonants may show up in various forms, e.g. вхожџ. We shall write the basic form of the root in capital letters (ХОД) and its variants in small letters (ход-, хож-, etc.).
The vowel may also show up in various forms, just as vowels in English roots do. For example, the Russian root СЕД 'sit' shows up with the vowel -а- in садќться and with the vowel -я- in сїдут and with the vowel -е- in сљсть (cf. the cognate English root SET in set, sit, sat, seat,...).
Russian and English share these features because of their common Indo-European heritage. English has also borrowed a substantial number of words from Latin and French, whose roots sometimes look more like Russian roots than the native English roots do. For example, the English root SET corresponds to Latin SED (as in sedentary, session, preside, president), which is more like the Russian root СЕД.
It will be helpful for you to draw upon this knowledge you have of Latin roots in your English vocabulary when you learn Russian words; for example the relationship of the Russian word председђтель 'chairman, president' with English sit may not be immediately obvious, but its root -сед- is clearly related in form and meaning to the latinate English word president.
There are two reason why you should develop an ability to identify roots in Russian words: (1) it may help you guess the meaning of a word you never met before, and/or (2) it may help you remember it once you've met it.
Even if a Russian root has no counterpart in your native or latinate English vocabulary, it is worth learning its forms and knowing the regular alternation of consonants in Russian. For example, if you know the word кнќга and if you know that г may alternate with ж, then you should be able to guess the meaning of related words such as кнќжный магазќн 'book store', кнќжный язІк 'bookish language', and чљковая кнќжка 'check book'. In short, a knowledge of roots and consonant alternations helps you expand your vocabulary considerably.