Read the manpage of sh-posix(1) for details. Relevant portion posted below:
sh-posix(1) . . % . file [arg]...
(period) Read and execute commands from file and return. The commands are executed in the current shell environment. The search path specified by PATH is used to find the directory containing file. If any arguments arg are given, they become the positional parameters. Otherwise, the positional parameters are unchanged. The exit status is the exit status of the last command executed.
The dot-space-filename (whether or not the filename contains a relative path of "./" means that you want to *source* or read the contents of the file into your *current* environment without starting a new shell (and thus losing the variable values).
Sourcing is done to pass environmental variables defined in a file into the environment of another:
# cat mythings #!/usr/bin/sh export NAME=Den
# cat someshell #!/usr/bin/sh . ./mythings echo "I am ${NAME}" exit 0
To see the difference, run 'someshell'; then change the line that reads:
Typically scripts are not sourced and need to be executable. Files that are sourced are typically the shell env files (for aliases and functions) and just need to be readable.
