The series of posts related to Lisp functions that the newcomer arriving from C++ might not have encountered continues with the declaim macro.
The declaim macro is generally used as a top-level form, and has the effect of modifying the behaviour of the compiler. Common uses include setting optimization and debugging states, or declaring that a function ought to be inlined where it is invoked. It might also be used to indicate that a top-level variable is to be made special. You will have seen me using the optimization settings in some earlier code examples, for instance here.
The new programmer is likely to be interested in this primarily for the compiler optimizations. Variables being declaimed as special have to do with dynamic vs. lexical binding, which might be a separate posting some time later.
It is important to note that it is unspecified whether the side-effects of declaim on compilation persist past what the C++ programmer would call a compilation unit. That is, if you compile a file with certain optimization settings using declaim, and then later compile another file, one that does not declaim specific settings, it is implementation-dependent whether or not the impact of the declaim macro is visible in the second compilation unit. This may be significant, particularly if you are writing library code.