Less-than or slanted equal to

Yes: same meaning, only the lower bar is tilted. In x ⩽ 10, x may be smaller than 10 or exactly 10. Mathematicians call this a non-strict inequality because equality is allowed. Its partner ⩾ means “greater than or equal to”, or “at least”.

Which form you see is a matter of typographic habit. Many French, Ukrainian and other Eastern European school books and papers use the slanted ⩽ ⩾, English-language books mostly use ≤ ≥, and Japanese schools write ≦ with a double bar. All say the same thing. Inside one document, stick to one style so readers don't suspect a deliberate difference.

The sign grew out of older marks. In 1670 John Wallis wrote a bar over the less-than sign; in 1734 Pierre Bouguer put a double bar under it. Printers later settled on the single bar, flat or slanted.

How do you type ⩽?

Use \leqslant in LaTeX, or the Unicode code 2A7D. No ordinary keyboard has a key for it, and in everyday text ≤ (or simply <=) is perfectly correct.