What do the symbols on the top left of the character select screen mean?

These symbols indicate certain events that you have completed with that character. Each character has its own post-it note to fill out, and each addition to the note unlocks an item or cooperative baby for the game. Marks with a red outline, as in the image below, indicate that the event was completed on hard mode. (A black outline for mark #8 indicates beating Greed Mode, while a red outline indicates beating Greedier Mode.)

With the release of Binding of Isaac: Afterbirth, there are 3 new additions to the post-it note. They are all in the last row of the note.

If we give them the following order:

1 2 3
4 5 6
7 8 9

Then they are:

1. Mom's Heart (or It Lives)
2. Isaac
3. Boss Rush (reached by defeating Mom within 20 minutes)
4. Satan
5. ??? (in The Chest)
6. The Lamb (in the Dark Room)

7. Mega Satan (reaching The Chest or the Dark Room with the Golden Key or similar)

8. Ultra Greed (final boss of Greed mode)

9. Hush (reached by defeating Mom's Heart/It Lives in less than 30 minutes)

While it was technically possible to do #7 prior to Afterbirth, it was not part of the post-it note, and isn't saved in the game's files. That means that even if you had earned it, you will have to repeat the accomplishment to earn the mark on the note again.

In addition, with the release of Afterbirth+, there's a new final boss whose completion also modifies the post-it note.

Beating Delirium will cause the post-it to become wrinkled. Beating it on Hard will cause the post-it to be bloodied around the edges.

Those symbols tell you, which "end boss" you have defeated with the current character.

With your Isaac, you defeated Mom's heart (heart), Satan in Sheol (upside down cross) and Isaac in the Cathedral (cross).

The red tint tells you, that you defeated them during hard mode.

In the spoiler, you'll see the rest of the symbols

1. Mom's heart | 2. Isaac | 3. Boss rush | 4. Satan | 5. ??? / Blue baby | 6. The Lamb

These symbols denote the specific points you've beaten with that character.

The red border tells you that the specific point was beaten in hard difficulty.

So on your image: Isaac has beaten

• Mom's heart on hard (the heart on the top line)
• Satan on hard (upside down cross on the right)
• Isaac on hard (cross on the left)

The Isaac one is an assumption though, as I haven't unlocked the cathedral yet.