Lets say you work on a company which has an employee working at home 'cause currently he is not at company's city (and will note be for the next 2 months). That's all fine, but it got complicated to ask the "sysadmin" to give him access to our server, his response takes so long and even this way the response propably will be a "no-no, sorry".
Well, at least some things got available to our employee and he can do its work.
It is raining and it is cold in the city and another employee would like to work at home; asking the sysadmin for access is not an option, so this employee explains the other employee that there is an option if he agrees to give access to your machine through SSH, so he could try some port forwarding through SSH and access the servers freely.
The slack employee then runs the following command on your machine:
ssh -L 8888:XXX.XXX.XXX.XXX:YYY
Where XXX.XXX.XXX.XXX is the IP address of the company's server you want to access, the "8888" is the port number at your machine which you access the company's services (you access the services through localhost:8888 and it will be forwarded to "XXX.XXX.XXX.XXX:YYY") and YYY, obviously, is the port. With that, you can access the services at company normally (actually, it should be a pretty slow, but it is better than rain and cold).
(For reference and pictures, please visit this site)
All the story was just to put that tip in a context, the slack employee will be at the office in the next monday :)