VLOOKUP(search_key, range, index, is_sorted) finds search_key in the first column of range and returns the value from column number index of that range, on the same row.
Set is_sorted to FALSE to ask for an exact match. If you leave it out, Sheets assumes the first column is sorted and may return a near match, which is rarely what you want with codes or names.
The lookup table often lives on another tab. Type the tab name and an exclamation mark before the range, Products!A2:D11, or click the tab while you write the formula and select the cells. Put quotes around tab names that contain spaces: 'Price list'!A2:D11.
When the key isn't in the table, VLOOKUP returns #N/A, and hovering the red corner of the cell says which value it couldn't find. Wrap the formula in IFNA(..., "text") to show a message instead.
Fill a lookup down a column with the range locked: Products!$A$2:$D$11.
=VLOOKUP(B2, Products!$A$2:$D$11, 3, FALSE)
=IFNA(VLOOKUP(B2, Products!$A$2:$D$11, 2, FALSE), "Check SKU")
Look up the unit price of the SKU in B2; show a message when a SKU is missing.