If my computer states all memory slots are currently in use, does not mean I am out of memory? Can I delete photos or something to have more memory?
Can I delete apps/photos to obtain more memory in my Mac?
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"All memory slots are currently in use" means that there is a RAM (random access memory) module installed in each slot that your Mac has. The greater the RAM number, the more files you can have open and work on at the same time. RAM is equivalent of a desktop workspace in a physical office.
The storage for your Mac (usually hdd or ssd) is the equivalent of a file cabinet or closet. If you need more storage space, to keep more photos and other documents stored on your computer, you need an hdd or ssd with greater capacity. These are commonly called startup drives or backup drives... storage.
So, you won't delete files to gain more memory (RAM)... you would delete files to gain storage space on your drive(s).