Doubt about current direction in CircuitLab
simulate this circuit &ndash; Schematic created using CircuitLab
Figure 1. (a) As drawn. (b) A more usual power flow. (c) Alternative configuration.
The confusion may be caused by CircuitLab's default current source configuration which is (arguably) upside down for this simple circuit. If we follow the current flow (red arrow) in Figure 1a we can see that it is going up through R1 and therefore the top terminal must be at a lower potential than the bottom.
Figure 1b shows a more typical schematic with clockwise current flow and positive rail at the top.
Figure 1c shows a more typical negative current source configuration.

Why is it a negative value? What convention does it use?

From CircuitLab Basics:

Ground
Every voltage in CircuitLab is calculated relative to the ground (GND) node, which is by definition at 0 volts. This means that every circuit has to have at least one GND element, or the circuit will not simulate.
The concept of a ground in a circuit simulator is similar but not identical to the concept of an electrical ground in the physical world. In real life, ungrounded battery-operated circuits work just fine, because to the circuit, only relative voltages matter. However, inside a circuit simulator (or even when solving a circuit on paper!), we have to pick one node to be our reference in order to calculate voltages at other nodes.

While this doesn't directly answer your question it does give a clue that each circuit element node will either have current in or out of it depending on the nodes relative voltages. In your example the top of the resistor is at lower potential than the bottom so current must be flowing out of it and hence the negative sign.