How to make a title appear on a player screen on touching a block

Since it wasn't clear what "Touching" a block means, I choose "Land on" as the solution

First, let us create a scoreboard to track which was the last stepped on block

/scoreboard objectives add last_stepped dummy

Here is the first command, let us call it [If statement], just to reference it later in this answer.

/execute as @a unless score @s last_stepped matches <N> at @s if block ~ ~-1 ~ minecraft:<YourBlock> run <Result>

Where [@a] is the players you want to target (You can learn more about target selector here), [last_stepped] is the name of the scoreboard we created before, [N] is the ID we gave the block (We will choose an ID for each block later, this isn't any ingame actual ID), [YourBlock] is the block he has to stepon, and [Result] is our [/title] command or our [/scoreboard set] (Shown later).

This translates to "Execute as every player, if the block right bellow each player (Here, @s refers to self, or each player previously targeted, in this case, by the [as @a]) is [YourBlock], unless the last stepped block is the one we are looking for now, if so, run [Result] for each player"

(In case you are not familiar with command blocks) Set each command block as the following:

For each block you want to track, we will need two [Results]:

/execute <If statement> run title <your title command>

/execute <If statement> run scoreboard players set @s last_stepped <N>

Let me leave two examples here:

/execute as @a unless score @s last_stepped matches 1 at @s if block ~ ~-1 ~ minecraft:gold_block run title @s title {"text":"Level 1","color":"red"}
/execute as @a unless score @s last_stepped matches 1 at @s if block ~ ~-1 ~ minecraft:gold_block run scoreboard players set @s last_stepped 1

/execute as @a unless score @s last_stepped matches 2 at @s if block ~ ~-1 ~ minecraft:iron_block run title @s title {"text":"Level 2","color":"red"}
/execute as @a unless score @s last_stepped matches 2 at @s if block ~ ~-1 ~ minecraft:iron_block run scoreboard players set @s last_stepped 2

For the use of the OP:

/execute as @a unless score @s last_stepped matches 1 at @s if block ~ ~-1 ~ minecraft:polished_diorite run title @s title {"text":"Level 1","color":"red"}
/execute as @a at @s if block ~ ~-1 ~ minecraft:polished_diorite run scoreboard players set @s last_stepped 1
/execute as @a at @s if block ~ ~-1 ~ minecraft:red_carpet run scoreboard players set @s last_stepped 2