Support/resistance levels are barriers that could force price trends to change direction.
These levels can take many forms : straight lines (horizontal or inclined), curved lines, indicators (moving averages, Fibonacci retracements), patterns, etc.
The more times a support/resistance level is tested by the price, the weaker it becomes.
You can identify support/resistance levels by connecting Highs with Highs and Lows with Lows, as displayed in the image below.
When the price is above
a specific level, that level is considered a Support
When the price is below
a specific level, that level is considered a Resistance