readLock.lock();

This means that if any other thread is writing (i.e. holds a write lock) then stop here until no other thread is writing.
Once the lock is granted no other thread will be allowed to write (i.e. take a write lock) until the lock is released.

writeLock.lock();

This means that if any other thread is reading or writing, stop here and wait until no other thread is reading or writing.
Once the lock is granted, no other thread will be allowed to read or write (i.e. take a read or write lock) until the lock is released.

Combining these you can arrange for only one thread at a time to have write access, but as many readers as you like can read at the same time except when a thread is writing.

Put another way. Every time you want to read from the structure, take a read lock. Every time you want to write, take a write lock. This way whenever a write happens no-one is reading (you can imagine you have exclusive access), but there can be many readers reading at the same time so long as no-one is writing.