Slashes are used as a replacement for the word 'or', but the spacing surrounding a slash, or lack thereof, changes the slash's intensity.
Let's take the following examples:
The books were red/green. I couldn't find anything there / be bothered to look.
As in the first example, when there are no spaces surrounding the slash and the slash is, instead, surrounded by two subjects/objects, we are to read that the slash refers to these subjects/objects directly. The first example, then, is to be read as ‘The books were either red or green. However, when there is, indeed, a space on either side of the slash, we are to read the slash as referring to the general concepts or ideas expressed in the sentence, rather than the immediate subjects/objects. As in the second example, we see that rather than opposing the words ‘there’ and ‘be’, as the slash would without spaces on either side of it, the slash causes the entire sentence itself to read: either ‘I couldn't find anything there’ or ‘I couldn't be bothered to look’.
Let's look at a slightly more complex example:
I didn't talk back at her / wasn't sarcastic to her just because I was embarrassed/angry…
Here, we see that two things are being expressed: the notions of arguing with someone and talking back to someone, and the sentiments of anger and embarrassment. The first slash, with spaces either side of it, tells us to read: either 'I didn't talk back at her’ or 'I wasn't sarcastic to her’; and the second slash, without spaces, implies: ‘I was either embarrassed or angry.’
A good way to think of this is that a slash means that one of the two (or more) things in opposition with one another could replace the other(s). 'Embarrassed' could replace 'angry' in the example above, for instance, and the sentence would read perfectly. It would not make sense, however, to replace the word ‘her’ with ‘wasn't’ and to expect the sentence to make sense: ‘I didn't talk back at wasn't sarcastic…’ This is because a slash with spaces either side of it refers to the ideas expressed and not just the words either side of it.
When two words are placed directly at either side of the slash, without spaces, we are to assume that each word can replace the other; whereas, when there is a space either side of the slash, we are to read that the slash applies to the overall, opposing ideas or notions expressed.
NB: Usually, with slashes with spacing on either side, although the ideas expressed change, the subject in question will always remain the same, as in the above examples where the subject remains 'I', in spite of the slash.