The language you typically hear in South Korean dramas, movies, TV shows, music, etc. is typically the dialect associated with the capital-metropolitan region. Thus, even if someone from the 함경 region in North Korea, the capital-metropolitan region in South Korea, or from 경상남도 at the southern most part of the Korean Peninsula say the exact same phrase, there will be certain assumptions and meanings derived based on the pronunciation, cadence, and tonal differences despite the words literally being the exact same.
Thus, as you learn Korean, it is worth simply being aware that not all of the language you hear will be interpreted or received the same way despite appearing identically. Similarly, variations are received differently as well. Of course, most classes in the Korean language teach the "standard" dialect which is from the central area of the Korean Peninsula, and most closely associated with the dialect of the capital (Seoul).
The Korean Peninsula is home to at least seven different dialacts (depending what source you reference, the numbers will vary slightly, as well as what constitutes a dialect), all of which are outlined in Figure 4.1.1. You will notice that the names of the major dialects are typically named after the region in which they are located (not very creative but logical). More importantly, however, is that there are often sub-dialects within those broader categories. Most Korean dialects are mutually intelligible phonologically, grammatically, and lexically, but there are a few that are almost impossible for others to understand such as the dialects (or languages) spoken on 제주도 or the 육진 area.
It's not necessary for you to know the exact differences other than that, broadly speaking, some dialects lack certain vowels found in "standard" Korean, numerous words are different, there can be differences in pronunciation of consonants, and so on. Further, verbal morphology for "common" verb endings that you will learn in standard Korean can be different. Some "dialects" are completely unintelligible (notably the language spoken on Jeju Island).
While although half of the South Korean population lives in the capital-metropolitan area and the dialect spoken there is referred to as 서울말, simply know that this is the prestige language in South Korea and therefore considered standard; the pronunciation, grammatical rules, certain vocabulary and morphology, etc., are considered the default and what is used in all formal manners of communication.
We simply recommend noting that there are always differences between and within "languages" and dialects, and that there is no "better" or "worse" version. This language prestige is separate (and in addition to) the high/low versions of vocabulary that emerge with native Korean vocabulary, Sino-Korean words, and in very modern, modern Korean, foreign (typically English) loan words. Move on to Section 4.2 to look at Lexical Prestige in more detail!