Sounds as if you need a relatively simple lookup, in which case:
You can do lookups in a couple of ways:
Depends on your preference as to which you use.
The easier version is the VLOOKUP, in which case you'd put this formula into column AG of the Homes sheet:
=VLOOKUP(
as soon as you get this far, Excel will help (a bit!) by telling you what you need next - the hovery text will say:
VLOOKUP(lookup_value , table_array , col_index_num , [range_lookup])
lookup_value is your postcode on Homes sheet
table_array is where you're looking it up, which is columns G to L of your Ratings sheet - the limitation of VLOOKUP is that the column you're looking up (postcode on Ratings sheet) has to be to the left of the one you want to know about (rating on Ratings sheet) which fortunately for you, it already is. It also has to be the left-most column, which is why we're selecting columns G to L, not A to L, so that column G (postcode) is on the left of what we're looking at.
col_index_num is the number'th column you want back. So G = 1, H = 2, I = 3, J = 4, K = 5, L = 6. So you want 6 since you want what's in column L.
range_lookup is optional but actually important. FALSE means it'll do an exact match, whereas TRUE (which bizarrely is the default setting) means it'll do an approx match. If your postcode is 1245, you don't want it to find 1240 instead if 1245 isn't there, so you want FALSE here.
So that gives you, if you're typing into cell AG2:
=VLOOKUP(O2 , 'Ratings Data'!G:L , 6 , FALSE)
That will bring back the rating for the postcodes on Homes tab. If it can't find the postcode, you'll get an NA error, which you can deal with using IFNA() or IFERROR() around your VLOOKUP. You also might want to use $ signs so that if you copy the formula elsewhere later it still looks at columns G to L each time.
eg
=IFERROR(VLOOKUP(O2 , 'Ratings Data'!$G:$L , 6 , FALSE),"Not found")
INDEX(MATCH()) works similarly and techies tend to prefer it, but most non-techie people I work with find it a step too far to learn and prefer to start with VLOOKUP which does the job pretty well 99% of the time for most normal work applications. If you're one of the techie people then search google for INDEX MATCH and there are probably some decent explanations ;-)
=INDEX($Return$Range,MATCH(LookupCellRef,$Lookup$Range,0))