In very layman terms,

LDFLAGS refers for linker flags and is often user defined libraries
CPPLAGS is used by the preprocessor and is often the include directory

For example, if I’m compiling multiple LDFLAGS and CPPFLAGS which is required by guile-2.2.4

# ./configure --prefix=/usr/local/guile-2.2.4 LDFLAGS="-L/usr/local/libtool-2.4.6/lib -L/usr/local/gmp-6.1.0/lib" CPPFLAGS="-I/usr/local/libtool-2.4.6/include -I/usr/local/gmp-6.1.0/include"