If you put a hyphen between two keywords, Google is likely to treat them as a single "unit", whereas a space is just a separator.
Perhaps an example will make this clearer.
Imagine 2 web pages:
A) Title: 15-inch laptop <- Google sees two keywords: "15 inch" and "laptop"
B) Title: 15 inch laptop <- Google sees three keywords: "15", "inch" and "laptop"
Now if you search for:
15 laptop
Then B) has a chance of showing up (a tiny chance, but still a chance) because Google indexed the keyword "15" independently of other keywords.
However, A) likely won't show up because Google indexed the keyPHRASE "15 inch" i.e. the hyphen acted to glue the two keywords together, as far as Google sees things.
At the same time, there is a tiny chance that Google will prefer A) for a search for "15 inch laptop" because it contains the keyphrase "15 inch" not just the keywords "15" and "inch".
I have to use the word "likely" because Google applies fuzzy matches to searches, i.e. it goes beyond the original intent of a search to try and widen the number of relevant results it returns. And it is possible - though unlikely - that 15-inch could get fuzzy matched and show up for searches that don't contain the exact expression "15 inch".