Commentary for Mikvaot 2:1
הַטָּמֵא שֶׁיָּרַד לִטְבֹּל, סָפֵק טָבַל סָפֵק לֹא טָבַל, אֲפִלּוּ טָבַל, סָפֵק יֶשׁ בּוֹ אַרְבָּעִים סְאָה סָפֵק אֵין בּוֹ. שְׁנֵי מִקְוָאוֹת, אֶחָד יֶשׁ בּוֹ אַרְבָּעִים סְאָה וְאֶחָד שֶׁאֵין בּוֹ, טָבַל בְּאַחַד מֵהֶם וְאֵינוֹ יוֹדֵעַ בְּאֵיזֶה מֵהֶן טָבַל, סְפֵקוֹ טָמֵא:
Someone who went down to immerse [in a <i>mikveh</i>], and there is an uncertainty as to whether he immersed or not, or an uncertainty as to whether it contained forty <i>se'ah</i> [a specific unit of volume] in it or it did not; or if there were two <i>mikva'ot</i> [gatherings of water collected by natural means and immersed in for purification], one with forty <i>se'ah</i> in it, and one without, and someone immersed in one of them but he does not know in which one he immersed, its uncertainty is impure [i.e. the case is uncertain and resolved as impure].
Bartenura on Mishnah Mikvaot
English Explanation of Mishnah Mikvaot
If it is doubtful whether he did immerse himself or not;
And even if he did immerse himself, it is doubtful whether the mikveh contained forty seahs or not;
And if there were two mikvehs, one containing forty seahs but the other not containing forty seahs, and he immersed himself in one of them but he does not know in which of them he immersed himself,
In such a doubt he is unclean.
Today's mishnah deals with a case of an impure person who immersed himself in a mikveh in order to become pure, but it is unclear whether he did so in a manner that would purify him. In all such cases he is not considered clean. The reason is that his last determinable status was unclean. Therefore, he remains unclean until we can be sure that he is actually clean.
The mishnah is pretty straightforward, so there is no explanation below.