Nationalism is an ideology rooted in the idea that the "nation" has fallen and/or is currently under attack, and must be restored to an imagined time of past glory.
Hearkening back to an imagined past usually means being socially conservative (anti-LGBT, anti-abortion, pro-religion (so long as it is the specific religion belonging to "the nation"), pro-patriarchy, etc), and so liberties, protections and rights gained by social movements are rolled back.
The enemies of the nation are unspecified, and are generally whatever useful scapegoat is available. In the UK, nationalists have usually pointed to the EU, EU immigrants, and refugees coming from Libya and Syria. They can also be political enemies. Judges who ruled that parliament must have a say in Brexit were named as "enemies of the people" by right wing press. It's often imagined that these groups are in co-ordination with one another, even if the only links are surface level and tenuous.
The "nation" or "the people" are much more narrowly defined. It will deem ethnic minorities, notable categories of immigrants, and religious minorities as not truly being a part of the nation. They can be born there, live their whole lives there and build families and businesses there, but because of their background, they'd be "not really American/British/nationality". Their presence is considered detrimental, their influence in society - no matter how small - is considered to be foreign influence and something to be resisted and stomped out. Because immigrants tend to accept lower pay and working conditions, they're often labelled as being lazy or dirty, and this can sometimes be used to suggest that poverty and poor health are inherent to these groups, and that they are the root of the problems.
The problems are by now, I hope, self evident. Decades of women's and LGBT rights are placed into question, societal problems are scapegoated onto minorities instead of actually being dealt with (thereby exacerbating them and making them worse), political discourse is degraded to the point of theatrics, and you lay foundations which - given the right circumstances and a charismatic and popular leader to legitimise the conspiracies that make their policies possible - can lead down the path to ethnic cleansing and genocide.
It was ethnonationalist conspiracy theories about "globalists" (a white nationalist term, often used interchangeably with the word "Jews") that fuelled all three of the incidents of political violence in the US in the last week. These conspiracies are emboldened and given legitimacy at times when they see the government as being on their side, and their actions as being helpful to the cause of restoring their "nation".
I know that's a lot to read, but I hope it's a good starting point for you.