dual boot - How to give Ubuntu 16 more space

I know this question was answered a thousand times but I can't find a solution. So here's what I have:

128Gb SSD with Windows 10 installed
1Tb HDD with a partition of about 80Gb that I used to install Ubuntu 16 (I am still asking myself why I didn't give it more space in the first place).

I now need to give Ubuntu more space from the HDD. I tried unallocating some space from Windows (5Gb) and while in Ubuntu I tried to follow the instructions here: How to give Ubuntu 14.04 more disk space but I can't unmount /dev/sda2.

How can I give Ubuntu more space then? Also what are exactly /dev/sda2, /dev/sda3, /dev/sda5 and /dev/sda6? I thought Ubuntu had only one partition. Which one should get extended?

Thank you

Where to allocate space

Your comments have already cleared that you need to run gparted from a live session so that it won't be mounted when re-partitioning the drive.

Looking at your image, Ubuntu is installed on partition /dev/sda2. That is also the partition where you are critically low in space. So you should assign the unllocated space to that partition where it's needed.

What are exactly /dev/sda2, /dev/sda3, /dev/sda5 and /dev/sda6?

Ubuntu only uses one partition as you thought. However a default installation will make a swap partition for increased performance. It's not a requirement. Also, there is an option to separate partitions for specific folders. People commonly create a separate partition for their home folder (/home). This isn't a requirement either, but is done for convenience for the occasions for reinstalling Ubuntu or moving to a different system and being sure to maintain their personal space untouched.

/dev/sda2    - The root installation of Ubuntu
/dev/sda3    - A primary partition that can be devided into logical partitions.
               The logical partition functions just like regular partitions.  This
               overcomes the 4 partition limit of a MBR hard drive.
/dev/sda5    - A swap partition.  This is a partition that used to function as if
               it were RAM.  It allows RAM to be swapped to the hard drive, freeing
               up RAM for other tasks.
/dev/sda6    - Your Home folder.  Setup for your personal space.