Markets are fully determined by the supply and demand of the product being traded and how this changes over time.
If there’s a shortfall in supply, this can cause surging prices and disruptions for consumers, who either have to overpay for their products or not get them at all.
A shortfall in the supply of beer some time
back caused a dramatic increase in price. The situation has stabilized
but seeing that this happened once may lead us to worry about it
repeating.
The supply and demand gauges use a set of heuristics to assign a score (0–5) to the supply and demand situation for each product.

This is a predominantly backwards looking measure (although it may take into account that eg. beer is demanded by commoners, who are more transparent and predictable than other market participants), and not able to predict the future any more than you are by looking at the market history.
The real value offered by these gauges is in how they summarize the situation and can help getting an overview of the situation across the large number of markets and production methods in Mercatorio.
Say you’re looking to get into forging and you’re deciding between the following production methods:

The gauges on the right look familiar, and rightfully so. Taking a closer look at the the tentatively most profitable production method, gather resin 1, reveals what could become a problem for us.

The production method estimated profit gauge inherits its rating from the supply and demand of respectively inputs and outputs. In this case it’s telling us that the estimate for the sales price of resin is “doubtful”, which in turn puts the profit estimate in question as well.

A closer look at the local resin market confirms this; there hasn’t been any trades for a long time and there’s currently no demand for resin in our town, meaning we’re unlikely to make much profit from this.
We can obviously immediately see this with a glance at the market history, but being being able to filter out doubtful estimates before even looking at the production methods speeds up this process immensely, and can help new players get their bearings in the market environment.
Read more about the market mechanics and price elasticity of markets, or go to Mercatorio.io to play and join the discord server to meet other players, plan strategies and strike deals.