What the left arrow means
The left arrow ← (officially leftwards arrow, U+2190) usually means "back" or "previous": the back button in an app, "← older posts" on a blog, or undoing a step. The hooked ↩︎ is the classic "reply" or "return" arrow, and 🔙 is the emoji version of a back sign.
In pseudocode and some programming languages, ← means assignment: x ← x + 1 reads "x becomes x + 1". APL uses ← this way, while R spells it with two plain characters, <-. In logic, ⇐ means "is implied by", the mirror image of ⇒. On keyboards, ⇤ and ⇥ appear on the Tab key, and ⌫ is the Mac Delete key.
Left arrows vs angle brackets
Several characters look like a left-pointing arrowhead but are not arrows. ‹ and « are quotation marks (guillemets) used in French, Russian and other languages. < is the less-than sign, which has its own page: the less than symbol. ◀︎ ◄ and ◂ are triangles, often used for "previous" in media players.
Pick the arrow by style: ← for plain text, ⟵ when you want a longer arrow, ⇐ for logic, ⬅︎ for something bold that may become a color emoji, and ⭠ or 🡐 for a clean geometric look. For the other direction, see the right arrow symbol.
How to type ←
On Windows, hold Alt and type 27 on the number pad to get ←, or Alt + 17 for ◄. On a Mac, open the Character Viewer with Control + Command + Space and search "left arrow". On phones, search "left" or "back" in the emoji keyboard.
In Microsoft Word, AutoCorrect turns <-- into ←, or type 2190 and press Alt + X. In HTML, use ← for ← or ⇐ for ⇐. In LaTeX, use \leftarrow (or \gets), \Leftarrow and \longleftarrow.