This page of the guide to Death Stranding explains the likes, how to effectively get likes and what bonuses you get by receiving new likes. This is one of the most important mechanics of the game, which can make the campaign much easier.
Likes can be obtained from NPCs and other players. In each of these cases, the methods of obtaining them and their application differ to a certain extent.
Likes from NPCs work as XP. You can receive them mainly by completing orders. When an order is finished, the game will display the mission summary screen. The number of likes received will depend on your performance, for example, if you avoided serious damage to the cargo or managed to complete an order within the time limit. These likes will increase the Connection Level in this location. In each location, the maximum Connection Level is 5 stars - advancing to the next levels gives you small rewards.
That's not all, because after each completed order, you can see the Courier Score screen. Received likes allow you to advance to higher levels in different categories, such as time or delivery size. Each time you advance 10 levels in a given category, you get a valuable bonus to Sam's stats. For example, you can increase the load capacity limit or maintain balance easier.
Likes received from other players strengthen your ties with them. The more likes players have sent between them, the more traces of their presence can be spotted. In short - by collecting likes, you can often come across objects and constructions of other players. Almost every object left by another player is useful in some way.
You can give likes to all constructions designs and signs left by other players - stand in the right place and press the touchpad. It is worth noting that you don't need to limit yourself each time to leaving only to a single like. You can quickly tap the touchpad to give another player a few dozen likes before the game temporarily blocks this option.
You can track the likes in Bridge Linksmenu available from the pause menu. In the later part of the game, you can choose from a list of specific players to establish Strand Contacts with them and further strengthen the relationship. You should choose these players who in the past left a lot of likes to your signs and constructions.