The Houston Astros have a lot of work to do this offseason, and adding new talent is only part of the equation. After they signed Nate Pearson towards the end of October, their 40-man roster was full, which means if they wanted to add anyone, it is going to require others to be removed. Houston did do a relatively expensive roster culling, which helped, but the upcoming Rule 5 Draft is only going to add to the roster headache.
The way the Rule 5 Draft, which occurs at the Winter Meetings every year, works is that minor leaguers who have been in the minors for certain lengths of time (exactly how long depends on their signing age) are eligible to be drafted by other teams unless their current clubs have them on their 40-man roster.