What is meant by inline elements?

Answer 1

If we're talking CSS, an inline element is an element (like a <div>, a <span>, or whatever you want) following the normal page flow, in opposition, for example, to block elements, which will go on a new line. (you can also choose inline-block, table, inline-table, etc., but i suggest you to look for documentation if you want to know more)

Answer 2

Inline elements :

b, big, i, small, tt
abbr, acronym, cite, code, dfn, em, kbd, strong, samp, time, var
a, bdo, br, img, map, object, q, script, span, sub, sup
button, input, label, select, textarea
They do not begin with new line.

________________
Block-level vs. inline

There are a couple of key differences between block-level elements and inline elements:

Formatting
By default, block-level elements begin on new lines, but inline elements can start anywhere in a line.
Content model
Generally, block-level elements may contain inline elements and other block-level elements. Inherent in this structural distinction is the idea that block elements create "larger" structures than inline elements.

The distinction of block-level vs. inline elements is used in HTML specifications up to 4.01. In HTML5, this binary distinction is replaced with a more complex set of content categories. The "block-level" category roughly corresponds to the category of flow content in HTML5, while "inline" corresponds to phrasing content, but there are additional categories.
____________
Block elements:
Just ckeck list here :D
https://developer.mozilla.org/en-US/docs/Web/HTML/Block-level_elements

_______________
Yep.Whole answer consists just from copied parts from different sites.
That takes few seconds to google it.