Linux Mint Update Manager explained

The Linux Mint Update Manager is designed to avoid (inexperienced) users from installing updates that require a certain level of experience and knowledge. The update Manager categorizes all updates into 5 levels. By default, level 1 to 3 are selected for updating, level 4 and 5 are deselected and won't be updated. Furthermore, each level can be set as safe and visible by selecting or deselecting these options. in addition to above, all updates are being checked by the developers team before being released.

Linux Mint is an operating system who aims to be an easy-to-use, elegant and comfortable platform with a high level of stability for both novice users as experienced users. This is the reason why Linux Mint chooses to deselect level 4 and level 5 updates granting permission for levels 1, 2 and 3 to be updated with nu limitations.

Levels 4 and 5 updates aren't as risky as some might pretend. For regular desktop users the security risks, created due to leakages in packages, are very slim and negligible. Also the risk of suffering from an unstable system after updating 
level 4 and 5 packages is very slim for regular desktop users. 
The 5 levels and their description are:

Level 1:
Certified packages. These packages has been tested or directly maintained by Linux Mint. Recommended to be updated when updates are available.

Level 2:
Recommended packages. All packages are tested and approved by Linux Mint. Also recommended to be updated.

Level 3:
Safe packages. All level 3 packages are not tested but considered to be safe. Recommended to be updated as well.

Level 4:
Unsafe packages. Updates classified as level 4 could have a potential negative affect on system stability. Deselected by default for safety and visibility.

Level 5: Dangerous packages. Updates classified as level 5 can have a negative affect on system stability depending on the system hardware and specifications. Also deselected for safety and visibility.
Although there are users who claim that level 4 and level 5 updates disrupt their systems when it comes to stability, there are plenty of users who select level 4 and level 5 for updating and not having any problems with their system after updating.

At the bottom of the Update Manager preferences menu, there are 2 options visible. The first option is selected by default. However, the second option is not selected. It is handy to select the second option as well.

Option 1 (selected by default):
Always show security updates

Option 2 (deselected by default):
Always select and trust security updates